network/onion: detect Tor liveness via socket close instead of polling

[eval-exec]

Feature Request

Is your feature request related to a problem? Please describe.

OnionService polls GETINFO uptime every 3 seconds to check if Tor is alive (

ckb/util/onion/src/onion_service.rs

Lines 130 to 149 in 3d0da7c

	self.handle.spawn(async move {
	let mut ticker = tokio::time::interval(tokio::time::Duration::from_secs(3));
	loop {
	tokio::select! {
	_ = ticker.tick() => {
	let uptime = tor_controller.get_uptime().await;
	if let Err(err) = uptime {
	error!("Failed to get tor server uptime: {:?}", err);
	drop(tor_server_alive_tx);
	return;
	}
	}
	_ = stop_rx.cancelled() => {
	info!("OnionService received stop signal, exiting...");
	drop(tor_server_alive_tx);
	return;
	}
	}
	}
	});

). This wastes control-port traffic and adds a 3-second blind window before CKB notices Tor is gone.

Describe the solution you'd like

React to the control-connection close instead of polling — either by detecting when the TcpStream closes, or by subscribing to SETEVENTS STATUS_CLIENT.

For reference, Bitcoin Core uses a libevent BEV_EVENT_EOF callback to detect the close instantly and then triggers a reconnect handler:

• Event callback: https://github.com/bitcoin/bitcoin/blob/faa18dceba1dd69703a252f751c233d227164689/src/torcontrol.cpp#L133-L148
• Reconnect handler: https://github.com/bitcoin/bitcoin/blob/faa18dceba1dd69703a252f751c233d227164689/src/torcontrol.cpp#L634-L652